Typically, during semiconductor processing, a (dry) plasma etch process is used to remove or etch material along fine lines or within vias or contacts patterned on a semiconductor substrate. The plasma etch process generally involves positioning a semiconductor substrate with an overlying patterned, protective layer, for example a photoresist layer, into a processing chamber.
Once the substrate is positioned within the chamber, it is etched by introducing an ionizable, dissociative gas mixture into the chamber at a pre-specified flow rate, while adjusting a vacuum pump to achieve a processing pressure. Then, plasma is formed when a portion of the gas species is ionized by collisions with energetic electrons. The heated electrons dissociate some of the gas species in the gas mixture to create reactant species suitable for the exposed surface-etch chemistry. Once the plasma is formed, any exposed surfaces of the substrate are etched by the plasma at a rate that varies as a function of plasma density, average electron energy, and other factors.
Conventionally, various techniques have been implemented for exciting a gas into plasma for the treatment of a substrate during semiconductor device fabrication, as described above. In particular, (“parallel plate”) capacitively coupled plasma (CCP) processing systems, or inductively coupled plasma (ICP) processing systems have been used commonly for plasma excitation. Among other or more specific types of plasma sources, there are microwave plasma sources (including those using electron-cyclotron resonance (ECR)), surface wave plasma (SWP) sources, and helicon plasma sources.
It is becoming common wisdom that SWP sources, which include a slot antenna, offer improved plasma processing performance, particularly for etching processes, over CCP systems, ICP systems and resonantly heated systems. SWP sources produce a high degree of ionization at a relatively lower Boltzmann electron temperature (Te) near the processing target (substrate). In addition, SWP sources generally produce plasma richer in electronically excited molecular species with reduced molecular dissociation. However, the practical implementation of SWP sources still suffers from several deficiencies including, for example, plasma stability and uniformity.
For a number of reasons, including charged ions and electrons recombining on chamber walls as they propagate from the source to the substrate, plasma density is often substantially non-uniform near the substrate. For ICP or CCP systems, such plasma density irregularity may be reduced by injecting a fraction of the process gasses into a region near the top of the chamber, and the balance of the gas through a ring near the substrate. This technique is somewhat effective when the electron temperature is sufficiently high to yield effective ionization and plasma-chemical reactions near the gas ring. However, since the average electron temperature in a SWP source that uses a slot antenna is relatively low, only molecules with weak chemical bonds can be cracked effectively near the gas ring. This limits spatial control of the plasma chemistry near the wafer and, therefore impacts the system application range. Therefore, an effective means to control the process plasma density in a surface wave plasma etch system with a slot antenna is needed.